Semrock Master Catalog 2018

Edge Steepness and Transition Width
Semrock edge filters – including our steepest RazorEdge ® Raman filters as
well as our EdgeBasic filters for application-specific Raman systems and
fluorescence imaging – are specified with a guaranteed “Transition Width.”
Transition Width = maximum allowed spectral width between the laser line
(where OD > 6) and the 50% transmission point
Any given filter can also be described by its “Edge Steepness,” which is the
actual steepness of the filter, regardless of the precise wavelength placement
of the edge.
Edge Steepness = actual steepness of a filter measured
from the OD 6 point to the 50% transmission point
Figure 1 illustrates Transition Width and Edge Steepness for an edge
filter designed to block the 785 nm laser line (example shows a U-grade
RazorEdge filter). Table 1 below lists the guaranteed Transition Width and
typical Edge Steepness (for 25 mm diameter parts) for Semrock edge filters.
All RazorEdge filters provide exceptional steepness to allow measurement
of signals very close to the blocked laser line with high signal-to-noise ratio.
However, the state-of-the-art E-grade RazorEdge filters take closeness to
an Extreme level.
The graph at the right illustrates that U-grade RazorEdge filters have a
transition width that is 1% of the laser wavelength. E-grade filters have a
Transition width that is twice as narrow, or 0.5% of the laser line!
Table 1
PRODUCT NOTE
Edge Filter Type
TECHNICAL NOTE
Guaranteed Transition
Width
(% of laser wavelength)
Ultraviolet (UV) Raman Spectroscopy
Raman spectroscopy measurements generally face two
limitations: (1) Raman scattering cross sections are tiny,
requiring intense lasers and sensitive detection systems just
to achieve enough signal; and (2) the signal-to-noise ratio is
further limited by fundamental, intrinsic noise sources like
sample autofluorescence. Raman measurements are most
commonly performed with green, red, or near-infrared (IR)
lasers, largely because of the availability of established lasers
and detectors at these wavelengths. However, by measuring
Raman spectra in the ultraviolet (UV) wavelength range, both
of the above limitations can be substantially alleviated.
Visible and near-IR lasers have photon energies below the
first electronic transitions of most molecules. However, when
the photon energy of the laser lies within the electronic
spectrum of a molecule, as is the case for UV lasers and
most molecules, the intensity of Raman-active vibrations can
increase by many orders of magnitude – this effect is called
“resonance-enhanced Raman scattering.”
Although UV lasers tend to excite strong autofluorescence,
it typically occurs only at wavelengths above about 300 nm,
Typical Edge
Steepness
(% of laser wavelength)
RazorEdge E-grade < 0.5% (< 90 cm -1 for 532) 0.2% (1.1 nm for 532)
RazorEdge U-grade < 1.0% (< 186 cm -1 for 532) 0.5% (2.7 nm for 532)
EdgeBasic < 2.5% (< 458 cm -1 for 532) 1.5% (8.0 nm for 532)
* except UV filters
Signal
Laser
Line
Transmission (% and OD)
100%
50%
10%
OD 1
OD 2
OD 3
OD 4
OD 5
OD 6
780 785 790
795 800
Raman
Signal
Laser
Line
Transition
Width
Edge
Steepness
Wavelength (nm)
Figure 1: Transition width and edge steepness illustrated.
Transmission (% and OD)
80 0 -80
-160 -240
100%
50%
10%
OD 1
OD 2
OD 3
OD 4
OD 5
Wavenumber Shift from 785 nm (cm –1 )
E-grade
~ 0.2%
U-grade
0.5% 1.0%
~ 0.5%
OD 6
780 785 790
795 800
Wavelength (nm)
Figure 2: Transition widths and edge steepnesses for
LP02-785RE and LP02-785RU filters (see page 94).
Autofluorescence
Noise
200 225 250 275 300 325 350 375 400
Wavelength (nm)
independent of the UV laser wavelength. Since even
a 4000 cm –1
(very large) Stokes shift leads to Raman
emission below 300 nm when excited by a common 266
nm laser, autofluorescence simply does not interfere
with the Raman signal making high signal-to-noise ratio
measurements possible.
An increasing number of compact, affordable, and highpower
UV lasers have become widely available, such as
quadrupled, diode-pumped Nd:YAG lasers at 266 nm
and NeCu hollow-cathode metal-ion lasers at 248.6 nm,
making ultra-sensitive UV Raman spectroscopy a now
widely accessible technique.
NIR Filters Mirrors Polarizers
Edge
Filters
Dichroic
Beamsplitters
Laser-line
Filters
Laser Diode
Filters
Notch
Filters
Lamp Clean-up
Filters
93
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